A finished video presentation is rarely one continuous scene shot from one camera. More commonly, video presentations are a series of short scenes assembled together through an edit process to form the final presentation. Unlike motion picture film which is edited by physically cutting and splicing film segments together, video material is edited by electronically cutting and splicing together segments of video signals.
A video switcher is an electronic device used by the video industry to execute a form of editing wherein video material from a variety of sources are assembled into a finished video presentation. Video material is provided to the video switcher in the form of input video signals from a variety of video sources, such as video tape recorders, optical disk plays, and video cameras. The video switcher typically selects for presentation as an output video signal one or more of the input video signals from the available video sources. This selection is accomplished by switching from one input video signal to another signal, with the time of occurrence of the switch operator. This output video signal is utilized in a variety of ways, such as recording by a video tape recorder, displaying on a video monitor, or broadcasting to viewers.
This switch between input video sources can take many forms and often involves several different switching techniques. The most straight forward of the switching techniques is the simple switch between different video signal sources. In such switchers, the output of the switcher is instantaneously switched between the input video signals and only one of the input video signals is present at any one instant at the output of the switcher. To a viewer of the output signal displayed on a monitor or television receiver, the display changes instantaneously from one input video signal to the other different input video signal.
Another technique of effecting a switch between video sources is called a dissolve. A dissolve is a gradual transition between different video signals, with the different video signals appearing simultaneously in the output provided for display during part of the transition interval. The gradual transition is accomplished through adjustable video signal gain controllers, commonly variable attenuators, and a signal combiner, which additively combine and present to an output the gain adjusted video signals provided by the gain controllers. Usually, a dissolve begins with the output video signal being entirely composed of a first input video signal. At a rate determined by the operator, the gain on the first input video signal is reduced to zero, while the gain on a second input video signal is raised from zero to the level originally set for the first signal. To a viewer of the output signal displayed on a monitor, the picture based on the first input signal appears to dim, while the picture based on the second input video source appears very dim at first and then appears to brighten until only the picture based on the second input video source is visible. At the midpoint of the switch, both signals are at an equal gain of one half and the pictures resulting from the displays of the signals are both visible simultaneously on the monitor, each at one-half its normal brightness.
Yet another technique of effecting a switch between video sources is called a wipe. A wipe is created by the generating a control signal which is used by a video mixer to switch between input video signals on a line by line basis, such that a line of output video may be composed of parts of both input video signals. This control signal is usually based on the output of ramp generators which are processed by a ramp combiner, which manipulates the ramps and then combines them so as to form a variety of control signals. During a wipe, both input video signals are present in the output video signal. Unlike a dissolve, where both complete input signals, at reduced gains, are additively combined to form the output video signal, in a wipe both signals are used at full gain, but only part of each signal corresponding to spatial segments of a display are present in the output video signal. Generally, the input video signals do not coexist in the same display spatial segment corresponding to the part of the output video signal. This concept is better understood from the standpoint of viewing the display of the output signal on a monitor. The wipe often begins with a first input video signal, when displayed on a monitor, spatially occupying the entire display area of the monitor. As the wipe progresses, the second input video signal displaces the first input video signal, spatially, in part of the display area of the monitor. The area displaying the second input video signal grows according to a selected pattern as the area displaying the first input video signal reduces according to a corresponding pattern, with the patterns determined according to operator inputs. The wipe is completed when the display area on the monitor is entirely occupied by a display of the second input video signal.
A control signal is used to prevent coupling to the output, input video signals areas that are not to be displayed on the monitor. This control signal is simply turned on or off. When on, the input signal is coupled to the output, and when off, the input signal is prevented from being coupled to the output. Because one of the input video signals is presented at the output video signal of the switcher, the control signal for the first input video signal is the reverse of the control signal for the second input video signal.
An example of a simple wipe is the vertical line wipe. When viewed on a monitor, a transition boundary, in the form of a vertical line between display areas on the monitor, defines the separation between the displays of the the first and second input video signals. As the vertical line wipe is executed, this transition boundary moves horizontally across the display area wiping from a picture based on the first input video signal to a picture based on the second video input signal. It is the form that this transition boundary takes, that determines what type of wipe is executed. For example, circle wipes have transition boundaries that form the shape of a box and circle, respectively. The greater the variety of wipes a switcher can execute, the greater its value.
These wipes have in common a transition boundary between the displays of the two video signals. This boundary is commonly called a border. A border is generated by creating control signals for each video signal, that comprise the output video signal, such that a region between the display areas on a monitor does not contain any input video signal. This region is filled by the switcher will a simple mono-color video signal, internally generated by the switcher.
There are a number of standard border types generated for use in creating wipe effects. The simplest type is an instantaneous transition line between two regions of a display of the video signals on a monitor. This can be referred to as a hard transition with no border. Another type is an instantaneous transition between the two regions with a colored area of variable width between them. This type of border is called a hard transition with a colored border. Yet another type of border has a dissolve-like border between the two regions. In this area both video signals are present but at reduced levels. This type of border is called a soft transition with no colored border. Soft and hard transitions and colored borders can be combined in various ways to produce three other border types. A soft transition on both regions to a colored border is called a halo border. A soft transition on one region to the colored border and a hard transition from the other region to the colored border is called a half halo. If the border is a closed geometric figure, such as a circle or a box, a soft outside transition, between the region surrounding the closed geometric figure and the border, and hard inside transition, between the border and the region inside the closed geometric figure, is called a half halo. Conversely, a hard outside and soft inside transition is referred to as a reverse half halo.
To produce this variety of border types has required complex, specialized circuitry. To the circuitry used to produce a hard color border, additional circuitry must be added to produce each desired effect. Such circuitry is expensive, complex to design and manufacture, and difficult to maintain. Perhaps more importantly, such complex specialized circuitry introduced gain and timing discrepancies because of their complexity and lack of commonality in processing. These discrepancies degrade visual performance.
These control signals are commonly generated separately. When the video regions that are controlled by these control signals are assembled, as when displayed on a monitor, they do not form clean transition boundaries. This produces wipes with bad transition regions, which is considered very undesirable to the user.
As a result of these problems, there is a need for an apparatus or method that is less complex. Such an apparatus or method desirably also can generate all the required border types to allow commonality in signal processing. Further, such an apparatus or method should provide for a good fit between the regions on a monitor display when the input video signals are mixed in the video mixer.